Certain cleaning or other low vacuum processes require the mixing of a gas/vapor between a plasma source (PS) and the wafer processing chamber. For example, a NF3/NH3 process can be used to remove SiO2 from Si. In this process, Ar is mixed with NF3 prior to being injected into a top PS.
Plasma is produced in the PS, which, in addition to some ionization of the Ar/NF3 mix, produces Fluorine radicals that are highly reactive. A relatively large diameter (˜25 mm to 50 mm diameter) with a relatively short length (˜100 mm to 300 mm) conductance tube is used between the PS and the chamber in order to minimize recombination of the Fluorine radicals.
In the example process, NH3 cannot be mixed with the Ar/NF3 prior to the PS because disassociation of the NH3 is undesirable. The NH3 is typically injected into a side of the PS to chamber conductance tube. However, injection into the side of conductance tube that has a low length to diameter ratio (˜2:1 to 12:1) will not provide effective mixing and thus, will lead to an uneven distribution of the NH3 on the wafer being processed. Uniformity of the distribution of the NH3 over the wafer is required for a robust and repeatable process.
Issues arising in the example process are attributable in part to: (1) insufficient diffusion time to ensure complete mixing; and (2) preservation of streamlines at required pressures. Mixing time is a function of a length and a diameter of a tube in which the gases flow viscously at a given velocity. To achieve complete mixing the tube must be long enough so that the time of the viscous flow from inlet to outlet exceeds the time for diffusion across the diameter. Generally, the tube length to tube diameter ratio may exceed 20:1 to achieve complete mixing. However, given the fact that the diameter must be large to preserve the formed F radicals, a 20:1 length to diameter ratio may not be feasible.
Streamlines are capable of affecting mixing of gases. At particular pressures, if streamlines of a flowing gas remain undisturbed, a flow of introduced gases may not be able to be properly mixed. For example, pressures resulting from gases leaving the PS range between 1 to 10 Torr. At these pressures, the flow of gases is generally laminar.
As a result, it is desired to create a system in which a gas is sufficiently mixed as well as minimization of radical/ion recombination from a plasma source.